An MEMS microphone is an acousto-electric transducer including an electret film having semi-permanent polarization which is manufactured by embedding electric charges through an electretization process, thereby eliminating a need for a direct current bias of a condenser. The electret film is produced by injecting electric charges into a dielectric film and fixing the thus-injected electric charges. An electric field formed by the injected charges causes a potential difference across electrodes of the condenser. As used herein, the term “electretization” means a process for injecting electric charges into a dielectric film so as to fix the electric charges in the dielectric film (i.e., so as to allow the dielectric film to retain the electric charges), and a term “retained charge amount” means an amount of fixed electric charges.
An MEMS (Micro Electromechanical System) microphone manufactured by processing a silicon substrate using a semiconductor processing technique has recently gained attention. The electretization method used for the MEMS microphone is a method for electretizing a dielectric film in an MEMS microphone chip, which is produced by micromachining a silicon substrate, mounted on a mounting board, or in the MEMS microphone chip per se. In the electretization method, one or more MEMS microphone chips are subjected to at least one corona discharge process using one needle electrode or wire electrode, thereby electretizing the dielectric film (Patent Document 1).
As a high-precision electretizing method, there is a method in which corona discharge is performed above a fixed electrode while a dielectric film of a condenser microphone is set to a ground potential and the fixed electrode is set to an electric potential different from the ground potential, thereby electretizing the dielectric film (Patent Document 2).
Patent Document 1: International Publication WO 2006/132193
Patent Document 2: JP-A-2007-294858